Article pubs.acs.org/est
Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Adsorption of PFOA at the Air−Water Interface during Transport in Unsaturated Porous Media Ying Lyu,†,‡,§ Mark L. Brusseau,*,§ Wei Chen,§ Ni Yan,§ Xiaori Fu,§ and Xueyu Lin†,‡ Institute of Water Resources and Environment and ‡College of Construction Engineering, Jilin University, Changchun, 130026, People’s Republic of China § Soil, Water and Environmental Science Department, Hydrology and Atmospheric Sciences Department, School of Earth and Environmental Sciences, University of Arizona, 429 Shantz Building, Tucson, Arizona 85721, United States Downloaded via UNIV OF SUSSEX on June 27, 2018 at 05:18:10 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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S Supporting Information *
ABSTRACT: Miscible-displacement experiments are conducted with perfluorooctanoic acid (PFOA) to determine the contribution of adsorption at the air−water interface to retention during transport in water-unsaturated porous media. Column experiments were conducted with two sands of different diameter at different PFOA input concentrations, water saturations, and pore−water velocities to evaluate the impact of system variables on retardation. The breakthrough curves for unsaturated conditions exhibited greater retardation than those obtained for saturated conditions, demonstrating the significant impact of air−water interfacial adsorption on PFOA retention. Retardation was greater for lower water saturations and smaller grain diameter, consistent with the impact of system conditions on the magnitude of air−water interfacial area in porous media. Retardation was greater for lower input concentrations of PFOA for a given water saturation, consistent with the nonlinear nature of surfactant fluid−fluid interfacial adsorption. Retardation factors predicted using independently determined parameter values compared very well to the measured values. The results showed that adsorption at the air−water interface is a significant source of retention for PFOA, contributing approximately 50−75% of total retention, for the test systems. The significant magnitude of air−water interfacial adsorption measured in this work has ramifications for accurate determination of PFAS migration potential in vadose zones.
1. INTRODUCTION The use of per- and polyfluoroalkyl substances (PFAS) in numerous industrial, commercial, and military applications has resulted in their widespread distribution in the environment (cf., refs 1−4). Numerous reports have demonstrated the presence of PFAS in soil and groundwater at fire-fighting training sites, manufacturing sites, and airports.1,2,4−9 Perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA) are two primary PFAS of concern. The PFOS and PFOA concentrations observed at many of these sites exceed the lifetime health advisory of 0.07 μg/L (combined total) that was recently issued for long-term exposure to PFOA and PFOS through drinking water.10 The risk posed by PFAS-contaminated sites as well as the effective remediation of such sites is mediated by the transport and fate behavior of PFAS in the subsurface. Sorption of PFAS by the solid phase of geologic media is one phase-distribution retention process that can have significant impact on PFAS transport and attenuation. The sorption behavior of PFAS in geomedia has been investigated in several studies conducted over the past decade (e.g., refs 11−23). There exist however additional retention processes that need to be evaluated for their potential significance to PFAS retention and transport in the subsurface.24 © XXXX American Chemical Society
Recent detailed assessments of PFAS occurrence and fate at field sites have demonstrated that vadose-zone sources are a primary subsurface reservoir of PFAS, serving as long-term contaminant sources to groundwater.25−27 Hence, it is critical to examine the retention and transport behavior of PFAS in water-unsaturated porous media. One primary retention process for unsaturated systems is adsorption at the air− water interface. Several investigations have demonstrated its significance for retarding the transport of both nonsurfactant28−32 and surfactant constituents (e.g., refs 33−41). PFOS, PFOA, and related PFAS are surfactants and will accumulate at air−water interfaces. Research conducted for chemical and water-treatment applications has illustrated the strong air− water interfacial activity of PFOS and PFOA (e.g., refs 42−48). Thus, it is anticipated that adsorption to air−water interfaces may be a significant retention mechanism for transport of select PFAS in the vadose zone.24 However, to our knowledge, the specific impact of air−water interfacial adsorption on PFAS Received: May 2, 2018 Revised: June 6, 2018 Accepted: June 14, 2018
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DOI: 10.1021/acs.est.8b02348 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology Table 1. Measured and Predicted Retardation Factors expt 0.35 mm sand sat 1 2 3 4 5 7 8 9 10 11 12 1.2 mm sand sat 1 2 3
water saturation
measured R
FAWIAa
predicted Rb
1 0.688 0.680 0.683 0.865 0.760 0.770 0.775 0.770 0.670 0.680 0.680
1.29 1.96 2.05 2.04 1.68 1.85 1.60 1.52 1.91 2.30 2.41 2.80
0.53 0.57 0.57 0.47 0.52 0.37 0.28 0.56 0.65 0.68 0.75
1.99 2.02 2.00 1.55 1.78 1.74 1.72 1.76
1 0.800 0.650 0.350
1.06 1.16 1.21 1.26
0.55 0.57 0.36
1.14 (1.12−1.16) 1.23 (1.18−1.28) 1.64 (1.49−1.82)
(1.83−2.18) (1.85−2.22) (1.83−2.20) (1.49−1.61) (1.67−1.92) (1.65−1.88) (1.62−1.85) (1.65−1.88)
notes v = 27 cm/h; Kd = 0.08 v = 37 cm/he R̅ 1−3c = 2.02 (1.96−2.07) COVd = 2.4%
v = 69 cm/h v = 137 cm/h v = 17 cm/h C0 = 0.1 mg/L C0 = 0.1 mg/L C0 = 0.01 mg/L Kd = 0.015
a FAWIA = fraction of total retention associated with air−water interfacial adsorption. b95% confidence interval in parentheses incorporating combined uncertainty in Kai and Aai. cMean of experiments 1−3. dCoefficient of variation for experiments 1−3. eMean pore−water velocity for all unsaturated-flow experiments except as noted.
0.7%. Two separate sets of measurements were conducted at different dates. In addition, a third set of measurements was conducted using a Kino automatic surface tensionmat employing the Wilhelmy plate method. Each sample was measured three times with a deviation between measurements of less than 0.3%. Surface tension was also measured for Na− PFOA in DI water using the De Nouy ring tensiometer. Surface-tension data for a C6−C11 homologous series of sodium perfluoroalkyl carboxylates in DI water were reported by Lunkenheimer et al.45 Analysis of these data, shown in Figure S1 of the Supporting Information (SI), provides an opportunity to determine Kai values for a series of PFAS. These data are used to evaluate the influence of chain length on the air−water adsorption potential. Surface-tension data for PFOS were reported by Vecitis et al.43 These data are used to determine a Kai value for comparison to the perfluoroalkyl carboxylate data. Two porous media were used for the miscible-displacement experiments. The first is a 16/18 mesh natural quartz sand ranging in size from 1.0 to 1.3 mm with a mean diameter of 1.2 mm. The second is a 40/50 mesh natural quartz sand with a mean diameter of 0.35 mm. These media, which are anticipated to exhibit ideal and relatively low magnitudes of solid-phase adsorption, were selected specifically to focus on the impact of air−water interfacial adsorption. The columns used in this study were constructed of acrylic to minimize interaction with PFOA and were 15 cm long with an inner diameter of 2.0 cm. Flow distributors were placed in contact with the porous media on the top and at the bottom of the column to help promote uniform fluid distribution and to support the media. Peristaltic pumps (BT100-02, Baoding Qili Precision Pump Co., Ltd., China) were used to provide fluid flow. As noted below, analysis of background samples collected from the column effluent revealed the absence of any interferences associated with the column or apparatus for PFOA determination. Methods. The miscible-displacement experiments were conducted using methods we have used previously in
transport in unsaturated porous media has not yet been investigated experimentally. The objective of this research is to conduct an initial investigation of the contribution of adsorption at the air−water interface to PFAS retention during transport in waterunsaturated porous media. PFOA is selected as the representative PFAS, and experiments are conducted with two natural sands of different particle diameter. Column experiments are conducted at different PFOA input concentrations, water saturations, and pore−water velocities to evaluate the impact of system variables on retardation and transport of PFOA. Independent measurements of air−water interfacial retention variables are used to determine predicted retardation factors, which are then compared to the measured values. Surface-tension data collected from the literature for a homologous series of perfluorocarboxylates are used to calculate air−water interfacial adsorption coefficients to investigate their dependency on chain length.
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MATERIALS AND METHODS Materials. PFOA (CAS# 335-67-1) of 98% purity was purchased from AIKE Reagent (China). Sodium perfluorooctanoate (CAS# 335-95-5) was also used in surface-tension comparisons. Sodium chloride (NaCl, 0.01 M) was used as the background electrolyte solution for all surface-tension and miscible-displacement experiments. Solutions were prepared using distilled, deionized water. PFOA input concentrations (C0) of 0.01, 0.1, and 1 mg/L were used for the miscibledisplacement experiments. The 1 mg/L concentration represents the upper range of PFOA concentrations reported for groundwater, with, for example, a value of >6 mg/L reported for Fallon NAS.6 The surface tension of aqueous PFOA solutions (with 0.01 M NaCl) was measured using a De Nouy ring tensionmeter (Fisher Scientific, Surface Tensiomat 21) following standard methods (ASTM D1331-89). The tensiometer was calibrated with a weight of known mass. Each sample was measured three times with a deviation between measurements of less than B
DOI: 10.1021/acs.est.8b02348 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
PFOA concentrations or other interferences for all experiments. Data Analysis. Measured retardation factors were determined for each miscible-displacement experiment by the standard method of calculating the area above the breakthrough curve. Their uncertainty is very small (total uncertainty for the measured R’s are
